Conventionally, studies have been conducted on a high electron mobility, transistor (HEMT) having an AlGaN layer and a GaN layer formed by crystal growth over a substrate, in which the GaN layer functions as an electron transit layer. The band gap of GaN is 3.4 eV, which is wider than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV). Accordingly, the GaN-based HEMT has high breakdown voltage, and is promising as a high breakdown voltage power device for automobiles or the like.
On the other hand, a body diode exists inevitably in an Si-based field effect transistor. The body diode is connected to a transistor to be in inversely parallel to the transistor, and functions as a free wheel diode in a full-bridge circuit method used for a high-power power supply. However, in the GaN-based HEMT, such a body diode does not exist inevitably. Accordingly, there has been proposed a structure in which a pn junction diode, which has a p-type layer and an n-type layer stacked in a thickness direction of the substrate, is connected to the GaN-based HEMT.
However, in the structure which has been proposed, a delay easily occurs in operation of the diode. Then, accompanying the delay, inverse electric current flows in the HEMT before the diode operates as the free wheel diode, and the power consumption increases. Further, when overvoltage is applied between the source and the drain of the HEMT due to the delay, the diode does not operate as a protective circuit.
There are a horizontal structure, in which a source and a drain are disposed in parallel to a surface of a substrate, and a vertical structure, in which a source and a drain are disposed vertically to a surface of a substrate, in structures of the high breakdown voltage power device. A current path is three-dimensional in the vertical structure, and therefore, a current amount per one chip is larger than the horizontal structure. Besides, the source electrode and the drain electrode are formed at a front surface and a rear surface of a substrate in the vertical structure, and therefore, an area required for the source electrode and the drain electrode is smaller than that of the horizontal structure, in which these electrodes are formed only at a front surface of a substrate. Further, a ratio of the electrodes per one chip in the vertical structure is larger than that of the horizontal structure, and therefore, a heat releasing property thereof is higher. Accordingly, practical use of the high breakdown voltage power device in the vertical structure is expected.
However, it is difficult to connect the above-stated pn junction diode to the GaN based HEMT in the vertical structure. Also, the above-stated problems are not solved even if the pn junction diode may be connected.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2009-164158                Patent Literature 2: Japanese Laid-open Patent Publication No. 2009-4398        
Non Patent Literature 1: Applied Physics Express 1 (2008) 011105                Non Patent Literature 2: Applied Physics Express 1 (2008) 021104        